Sparring in a combat sport is a totally different situation. In the OP's case, he's asked to practice a specific defense; in sparring, you defend (and attack) to exploit whatever situation arises, using whatever you think will work.

We are talking here about LEARNING, so no, situation is not different...
There are many kinds of sparring, one of them is to do predefined technique against countering opponent - can ppl doing such sparring learn something even if they can't do a technique at all?
It is kind of intermediary step between learning technique in a static situation(typical for aikido training) and dynamic application any technique against any attack....